A series of laser microbeam experiments will be conducted on mitotic chromosomes of various cell lines. Specific studies will be undertaken to: (1) elucidate further the possible mechanism(s) of chromosome loss, replacement, and stability; (2) determine the feasibility of using this technique for somatic cell genetic studies and studies on chromosome involvement in carcinogenesis; (3) further investigate the function and organization of the ribosomal genes and the nucleolus. The specific procedures employed in these studies are laser microirradiation, electron microscopy, light microscopy, and tissue culture.